When Did I Conceive?

Enter the first day of your last period and your average cycle length above, and the calculator will estimate your most likely conception date — typically about 14 days before your next period was due, whenever ovulation happens in your cycle. If you already know your due date instead, switch to the "Due date" tab and it'll work backward from there.

Use the "Due date" tab with your own birthday in place of a due date — babies arrive on their exact due date only about 5% of the time, so this gives an estimate rather than a certainty. If you know your birth parent's last-period date instead, the "Last period" tab will do the same calculation the other direction.

Your conception date is the day an egg was fertilized — the actual start of the pregnancy. It's usually a few days after ovulation, and about two weeks after the first day of your last period in an average cycle. Due dates, by contrast, are calculated from the last period date itself, not from conception.

It's an estimate, not a diagnosis. Ovulation timing shifts from cycle to cycle — even a "regular" 28-day cycle can ovulate anywhere from day 12 to day 16 or so — which is why we show a possible window rather than a single day. An early dating ultrasound (ideally in the first trimester) is the most accurate way to confirm timing.

Yes — run the calculator once using your last period date to see the estimated fertile window, then compare each candidate date against that window to see which one falls inside it. The tool doesn't store or compare dates for you, but the window itself is the reference point either date needs to fall within.

No — it only estimates when conception likely happened based on dates you enter; it can't identify or confirm paternity. If you're weighing two possible partners against a conception window, run each candidate date through separately (or check it against the fertile window above) to see which one falls inside the likely range — a paternity (DNA) test is the only way to confirm parentage itself.